The present invention relates to beverage containers, and more particularly, to self-standing plastic carbonated beverage containers with bases having legs providing foot surfaces to support the container.
Plastic containers, particularly blow-molded plastic containers for storing pressurized liquids, have assumed increasing importance in the beverage container market. Plastic containers have the advantage of being light weight, relatively inexpensive to produce, and are more resistant to breakage and other types of impact damage than are containers made of metal, ceramics or glass.
Typically, plastic containers are manufactured using a process primarily comprised of two molding operations. In the first step, a parison or preform is formed in an injection mold using standard molding techniques. During the injection molding process, liquefied plastic material is inserted into the mold and contacts the inner mold surfaces that are cooled by internally circulated water, such that the liquefied material solidifies into the desired shape of the preform. The resulting preform is generally tubular-shaped with a circular cross-section and has an open end and an enclosed end.
As a result of cooling the liquefied material to form the solid preform, the preform is extracted from the injection mold at a relatively cool temperature that is unsuitable for the second molding operation. Therefore, the preform must be heated to at least a minimum temperature such that preform becomes sufficiently ductile or stretchable to be blow-molded, as discussed below. The minimum required temperature is dependent upon the intrinsic viscosity of the preform material, which is a measure of the material's resistance to being formed or stretched. Thus, the greater the intrinsic viscosity of the resin, the higher the required minimum temperature to bring the preform to a state suitable for blow-molding. Further, the thicker that the preform is made, the higher the molding temperature should be as it is more difficult to stretch thicker material.
Ordinarily, the preform is transported through a heated area, such as a production oven, so that thermal energy is transferred to the preform to raise it to the desired minimum temperature. The preform is located within the oven for a period of time sufficient to raise the preform to the desired molding temperature. Therefore, the preform will be heated for a longer period of time if the intrinsic viscosity or thickness of the preform dictates that a higher forming temperature is required. Further, a thick preform must generally be heated for a relatively longer period of time, even if to achieve the same temperature as a thinner preform, as the greater amount of material requires more thermal energy to raise the temperature of the preform.
After heating to an appropriate temperature, the preform is placed within a blow mold. The blow mold has an internal cavity defined by wall surfaces that have been machined to the desired outer dimensions and shape of the molded container. Compressed air or another suitable pressurized gas is directed or "blown" into the hollow center of the preform such that the preform material stretches both radially outwardly and axially downwardly into contact with the mold surfaces. As the mold walls are cooled by internally circulating water, the heated material of the preform solidifies into a final shape provided by the mold walls substantially immediately upon contact with the walls.
Often, plastic containers are formed on a variation of the molding process called "stretch-blow molding". Stretch-blow molding is essentially the same basic process described above with the additional feature that a stretch rod is inserted through the center of the preform immediately before or after or simultaneous with the injection of the pressurized gas. The movement of the stretch rod facilitates the downward stretching of the preform toward the lower end of the blow mold.
In particular, the molding of the container base introduces several limitations into the manufacturing process. One limitation is that the larger the desired diameter of the finished base, the greater the gas pressure required to force the material to expand outwardly to reach the mold surfaces when the gas flow rate remains constant. However, the higher the pressure used to form the container, the greater the chance the force of the pressurized gas will cause a rupture in the container material, a situation referred to in the container-forming art as "blow-through". Blow-through tends to occur most often in the outer sections of the container base as the material is stretched further than at other sections of the container. Therefore, the higher the molding pressure used to form the container, the greater the required minimum thickness of the preform to prevent "blow-through" from occurring.
Further, as mentioned above, the greater the thickness of the preform used to make a given container, the higher that the molding temperature of the preform should be to enable the preform to stretch sufficiently during blow molding. The ability of the preform to stretch is most critical for forming the outer, lowermost portions of a container base as the preform must stretch the furthest distances both axially and radially to reach the mold surfaces that form these container portions. Another limitation is that, given only a specified amount of time for heating the preform, when the thickness of the preform is increased, the intrinsic viscosity of the preform material may be limited to below a maximum value so that the preform remains sufficiently stretchable to form the container. Thus, certain polymeric resins having a higher intrinsic viscosity may be unusable for making a container with a greater finished thickness or in a more time critical process.
Each of the above-discussed limitations to the container forming process affects what is referred to as the "process window", which is a set of process parameters that must be carefully controlled in order to produce commercially acceptable containers on a reliable basis. The factors included in the process window include the molding temperature of the preform, material viscosity, dwell time in the mold, pressure of the air/gas blown into the preform and, in stretch blow-molding operations, the stretch force of the rod exerted on the preform during the blow-molding process. Controlling the process window is critical for efficient manufacturing of the containers as the containers are produced in a high speed environment such that slight variations, minor modifications or aberrant fluctuations in any one of these parameters may lead to the fabrication of containers that are unacceptable.
When the specific configuration of the container is such that the range of acceptable values for any of the process parameters is decreased (e.g., by increasing the required molding temperature of the preform), the more critical it becomes to control these parameters, leading to a situation called a "narrow process window". With a narrow process window, there is little allowance for even slight changes to any of the process parameters. Therefore, the container-forming industry is constantly seeking new ways to "widen" the process window so as to increase the rate of production of acceptable containers.
Numerous types of known plastic containers, particularly for use in containing liquids at elevated pressures, are produced using the blow-molding process generally described above. These containers are generally of either two-piece construction, in which a separate base is attached to the remainder of the container, or a one-piece construction having an integral base structure. Referring to FIG. 1, a typical two-piece container 1 has a main container body 2 for holding the intended contents of the container 1 and a separate base member or cup 3 which is attached to the lower end of the main body 2 to enable the container body 2 to be supported in an upright position on a surface S. Each component 2, 3 of the container 1 is molded in a separate process and then the two components 2,3 are assembled together in a third, subsequent process, generally by gluing the base cup 3 to the container body 2. Typically, the container body 2 is transparent and made of polyethylene terephthalate ("PET") and the base cup 3 is formed of opaque high density polyethylene (HDPE).
Generally, the one-piece plastic container with an integral base is preferable as it requires less material and less processing to manufacture. Examples of one-piece plastic containers are found in U.S. Pat. No. 5,320,230 to Hsiung entitled "Base Configuration for Biaxial Stretched Blow-Molded PET Containers"; U.S. Pat. No. 5,353,954 to Steward et al. entitled "Large Radius Footed Container"; U.S. Pat. No. 5,484,072 to Beck et al. entitled "Self-standing Polyester Containers for Carbonated Beverages"; U.S. Pat. No. 5,549,210 to Cheng entitled "Wide Stance Footed Bottle with Radially Non-Uniform Circumference Footprint"; and U.S. Pat. No. 5,603,423 to Lynn et al. entitled "Plastic Container for Carbonated Beverages".
Referring now to FIGS. 2-4, a common type of one-piece plastic container 10 has a base 14 generally adapted from the base cup 3 of the two-piece container shown in FIG. 1. As best shown in FIG. 4, the base 14 has a cross-section formed generally as a barrel with an annular ring so as to be self-standing. One problem with the base structure is that the concave central portion 19 of the base 14 has the tendency to deflect or "pop" outwardly by the pressure of carbonation gas when the container 10 is filled with a substance such as a carbonated beverage. To prevent the outward deflection of the central portion 19, reinforcing ribs 24 were added to the base structure such that the base 14 is divided into several individual legs 16. The resulting base structure is commonly referred to as "petaloid" (i.e., resembling the petals of a flower).
More specifically, such petaloid bases 14 are typically formed of three or more legs 16 extending downwardly from the sidewall 12 that forms the main portion of the container 10. Each leg 16 is multi-sided or multi-faced and is formed of an outer side wall 17 extending generally continuously from the container side wall 12, an inner side wall 18 connected with a central portion 19 of the base 14 and two radially-extending and converging side walls 20A, 20B. An end wall 22 encloses the lower ends of the four side walls 17, 18, 20A and 20B and provides a foot surface 21 so that the container 10 may be placed in a "standing" position upon a surface S. Further, as discussed above, each adjacent pair of legs 16 is separated by a rib 24, such that the base 14 has a number of ribs equal to the number of legs 16. Each rib 24 extends between the side wall 12 and the central base portion 19 and has a generally arcuate shape.
By having legs 16 formed of a four distinct side walls and a separate enclosing end wall, regions of high stress concentration are formed. In particular, high stress concentration occurs in the base sections located at each inner corner of the legs 16, designated as region "I" in FIG. 3. The region I encompasses the intersection of four leg surfaces: the inner wall 18, one of the side walls 20A, 20B, the central base portion 19 and the proximal rib 24. Although this region, as with the central region 19, tends to have less biaxial orientation than other portions of the container 10 since less stretching of the preform occurs in this region during the molding process, the relatively high rate of stress failure of containers 10 in this area is primarily due to the geometric stress concentration arising from the intersections of the several surfaces. When the container 10 is filled with a pressurized substance, the walls of the legs 16, the ribs 24, and the central portion 19, deflect outwardly further at their respective central regions than at the relatively stiff regions of intersection with the various other wall portions. The deflection of these various wall portions cause sheer stress to be concentrated at the regions of intersection between the walls (in a manner analogous to a bending cantilever), which effect is multiplied by the convergence of several lines of intersection.
The base region I, as described above, is the area of the container 10 that is most likely to experience a failure mechanism referred to as "environmental stress cracking". Environmental stress cracking is the most common and most serious mode of failure for containers constructed of PET, such as the containers 10. Due to the stress concentration in region I arising from the structure of the legs 16 (as described above), the resulting magnitude of the stress experienced in this region of each leg 16 causes, over a period of several days or weeks, a gradual breakdown of the molecular structure of the PET material in the region I, initially causing one or more microscopic openings to form in the region I. Once an opening is formed, the stress concentration is further magnified at the opening such that the opening becomes greatly enlarged, leading to a catastrophic failure of the container 10.
A failure of a container 10 due to environmental stress cracking ordinarily occurs after a period of at least several days after the container 10 is filled with a pressurized substance, such as a carbonated soft drink. Therefore, the failure of the container 10 not only results in a loss of the container 10, but also a loss of the pressurized contents. Particularly when the contents of the container 10 is a quantity of a carbonated soft drink and the failed container 10 is stored with numerous other containers 10, the resulting spillage of the contents leads to a relatively labor intensive cleaning process to remove the spilled contents from the surrounding area.
Ordinarily, PET material is characteristically tough and durable such that failure of the containers 10 due to environmental stress cracking would generally not occur without the stress concentration introduced by the multi-sided structure of the legs 16. Environmental stress cracking is most likely to occur when the containers 10 are stored under conditions that are not optimal. Ideally, the containers 10 should be stored with the lowest feasible carbonation pressure and at the lowest temperature possible to minimize carbonation pressure. Clearly, by having a lower pressure, the stress in the walls of the container 10, such as in region I, will be minimized. Further, the containers 10 should be free of the lubricants that are used to facilitate handling of the containers 10 during the container-filling process. These lubricants, which are typically liberally applied to the containers 10 so as to have maximum effectiveness during the handling operations, contain chemicals which can cause PET material to break-down.
In reality, however, the ideal conditions are not generally attainable for the following reasons. Consumers prefer higher levels of carbonation in the beverages that they drink. Also, it is generally impossible or at least economically unfeasible to control the temperature of storage areas, such as warehouses or trailer trucks. Further, processes for removing the lubricants from the containers 10 are generally too costly to be implemented, such that the containers 10 are typically stored with a certain amount of the lubricant coating the base 14. Therefore, due to the presence of these factors, the resulting environmental stress cracking has led to an unacceptable number of failures of the prior art containers 10.
One container having a leg configuration that reduces the stress concentration effect of multi-sided legs is disclosed in U.S. Pat. No. 4,318,489 of Snyder et al. ("Snyder"). As shown in FIGS. 5-7, the Snyder container 110 has a base 114 formed of a plurality of bulbous or "spherical" legs 116 extending downwardly from a generally hemispherical base portion 114. Each leg 116 has a radially outermost wall portion 116a that is generally "vertically aligned" with the side wall 112 of the container 110 and the remaining upper end of each leg 116 intersects with the hemispherical portion 115, as best shown in FIGS. 6 and 7. Although the Snyder container 110 eliminates the multi-sided leg structure to thereby reduce stress concentration in the base region I (as described above), the configuration of base 114 introduces other deficiencies, as described below, that are not present in the typical container 10.
By having legs 116 that are bulbous or spherically-shaped, each leg 116 has only a relatively small foot surface 121. Therefore, when the Snyder container 110 is placed on a surface S, the container 110 is essentially supported on a plurality of points (i.e., the apexes of the surfaces 121) such that friction between the container 110 and the surface S is substantially less than with the common petaloid container 10. The minimal friction increases the likelihood that the container 110 will either tip over or slide rather than remain stationary relative to the surface S when subjected to an external force, which is particularly problematic for the handling of numerous empty containers 110, such as when the container 110 is located upon a tabletop conveyor (not shown) during a "bottling" or other container-filling operation.
Furthermore, as each foot surface 121 is located at approximately the center of the respective leg 116, the legs 116 should be located as far from the central axis 111 of the container 110 as possible so that the container 110 has a sufficient standing ring R. In general, the greater the standing ring of any container, the greater the container's stability and the less likely the container is to tip over during handling. This is due to the individual foot surfaces (e.g., 121) of the container being located further from the container's center of mass (which is located on the central axis 111), and thus each having a longer lever arm with which to resist a "tipping" moment arising from a force applied to the container. Therefore, the structure of the legs 116 having foot surfaces 121 only at about the middle thereof dictates that the legs 116 should located with the outermost edges 116a of each leg 116 vertically aligned with the side wall 112 of the container 110 for purposes of stability.
Another serious limitation of the Snyder container 110 results from the configuration of the legs 116 having an outer edge 116a "vertically aligned" with the side wall 112. By being "vertically aligned", the outer edge 116a of each leg 116 is thus located at the maximum distance from the center line 111 of the container 110. Therefore, when forming the legs 116, the preform material has to stretch to both the maximum radial and axial distances of the container 110, thereby causing the material in this region to thin to the extent that blow-through is likely to occur. Increasing the thickness of the preform to alleviate the excessive thinning necessitates increasing the pressure of the injected air so that the preform material stretches a sufficient distance to form the vertically-aligned outer edge 116 of each leg 116. However, the increased air pressure itself will likely cause blow through to occur. Therefore, the Snyder container 110 is only potentially produceable in a smaller size, such as of the now common "twenty-ounce" variety.
Furthermore, a problem that is common to both types of prior art containers 10, 110 described above is that, during formation of the container base 14, 114, the material forming the lower, outer edges of the legs 16, 116 (indicated in the drawings as region "O") undergoes greater stretching than at any other section of the container 10. This is due to the preform material in these regions having to be stretched both the greatest axial distance (as with the bottom surface of the base 14, 114 generally) and to stretch almost the same radial distance as the sidewall 12, 112. Due to the substantial amount of stretching of the material, if the preform is not sufficiently thick, the region O of each leg 16, 116 tends to become over-stretched and form an opaque section of material referred to as "pearled". Pearled areas are extremely thin and become easily wrinkled or dented, either outwardly from the internal pressure of the pressurized substance or inwardly from impact to the container (e.g., from being dropped). Further, pearled areas diminish the aesthetic appeal of the container 10, 110 to a consumer as there is the general expectation, particularly with carbonated beverage applications, that the walls should be generally transparent as with the glass containers that PET containers have replaced.
To eliminate the occurrence of pearling in the outer areas of the legs 16, 116, the thickness of the preform may be increased, with a corresponding increase in material costs. Another way to minimize the occurrence of pearling is to heat the preform for a longer period of time to increase the molding temperature so that the preform material is more ductile and thus less likely to over-stretch. The increase in heating time results in a reduced process window such that the rate of production of the containers 10, 110 is decreased.
From the foregoing, it will be appreciated that it would be desirable to have a container with an improved base that minimizes the amount of material necessary to manufacture each container. Further, it would be advantageous to provide a container having a design that is resistant to environmental stress cracking. It would also be desirable to provide a container having a sufficiently large foot surface area and/or standing ring so that the container has maximum stability to prevent toppling of the container, particularly during the manufacturing thereof. Furthermore, it would be desirable to provide a container with an improved base configuration such that the process window for manufacturing the container is maximized.